One of the most important cross-linked industrial polymers is vulcanized rubber used for tire manufacturing. Recycling of waste tire rubber has been a matter of commercial interest and scientific endeavors since the invention of the rubber vulcanization process by Charles Goodyear in 1839. Early attempts using rather simple thermo-chemical means were to recover the rubber as elastomer for subsequent use in molding operations by mixing with virgin rubber. But the modern synthetic rubber such as styrene-butadiene (SBR) is more resilient and difficult to devulcanize. Attempted procedures of tire rubber devulcanization have been extensively reviewed in the scientific literature (see for example: B. Adikari, D. De, S. Maiti, Reclamation and Recycling of Waste Rubber, Prog. Polym. Sci., 2000, 25, 909-948; M. Myhre, D. A. MacKillop, Rubber Recycling, Rubber Chem. Techol., 2002, 75, 429-474). Very often they involved the use of harmful and bad-smelling chemicals, such as mercaptans, disulfides, mercapto- or thio acids, peroxides, aliphatic or aromatic amines, phosphines, etc. Apart from being toxic and difficult to handle, such chemicals are not available commercially in sufficient quantities to sustain large-scale, million-ton recycling operations. Use of chemicals was described, for example, in U.S. Pat. Nos. 4,211,676, 4,305,850, and 6,387,965. Phase transfer catalysis was employed in the U.S. Pat. No. 4,161,464 to facilitate distribution of devulcanization agents inside the rubber matrix. Another piece of art described in U.S. Pat. No. 5,798,394 teaches us to use a suspension of alkali metals such as sodium in aromatic solvents for the surface devulcanization of finely ground tire rubber. These and many more methods described in the patent and scientific literature rely on the use of rubber crumb or other forms of finely particulate rubber in order to increase the mass transfer and the reaction rate. This is also the case for microbiological devulcanization methods such as one described in the U.S. Pat. No. 6,479,558.
U.S. Pat. No. 3,725,314 issued to Pelofsky in 1973 heralded the use of modern physical methods such as ultrasound to rubber recycling. The use of ultrasonic energy was further developed in the U.S. Pat. Nos. 4,599,711 and 5,284,625 as well as in the Japanese patent JP 62,121,741. Microwave irradiation was another advanced devulcanization method proposed in the U.S. Pat. No. 4,104,205. U.S. Pat. No. 7,629,497 assigned to Global Resource Corporation describes a multiple-frequency microwave method for recovery of oil and carbon black from scrap tires and other carbon-based materials. The energy transfer methods although rather clean and benign compared with the above mentioned chemical methods are too energy intensive, and their scalability is a matter of debate. In addition, microwave devulcanization is most effectively applicable to polar rubber such as nitrile-butadiene or chloroprene, but much less so for nonpolar natural rubber and SBR, which are the main components of automotive tires.
Conversion of non-dietary biomass into energy and chemical feedstock has been an active area of research and development since the emerging shortage of oil and gas was identified in the 1970s. Fermentation is a natural way to process plant biomass, but the lignin matrix renders lignocellulosic materials tough and resilient. Various methods have been used to unlock the lignin matrix and to facilitate bioavailability. Kraft process has been one of the first and most abundant technologies of biomass conversion. Although the technology is rather simple and uses cheap chemicals, it generates a lot of liquid wastes and is not environmentally friendly. Acidic or alkaline hydrolysis is another old method which is still widely in the art, as exemplified, for example, by U.S. Pat. Nos. 6,409,841 and 8,062,428. Delignification methods based on extraction with near- or supercritical water or alcohols are described in the U.S. Pat. Nos. 6,419,788, 8,053,566 and in the US patent application 2010/0043782. U.S. Pat. No. 4,644,060 teaches us how to increase bioavailability of fermentable polysaccharides by the treatment with supercritical ammonia. Dry gasification of biomass-rich materials is described in the U.S. Pat. No. 8,137,655. Implementations of almost all of those methods involve high temperatures (from around 200 to 750° C.; 400-1400° F.) or high pressures (up to 300 bar; 4300 psi) or combinations thereof. Another serious problem is the recovery and purification of the used reagents such as ammonia, alcohols, and water.
Cross-linked artificial polymers such as vulcanized rubber, polyurethane, epoxy, etc. constitute a significant part of the solid wastes generated by mankind, which consumption steeply grows every year. Natural polymers such as lignocellulosic biomass (wood, straw, sugarcane bagasse, switch grass, etc.), and protein-based biomass (wool, leather, etc.) are produced due to both natural processes and human agricultural activity. Environmental, economical, and human health concerns of the growing solid waste accumulation are well known: pollution of the environment through volatilization and leaching, fire hazard, breeding of harmful insect and rodent species, usage of significant tracts of land suitable for agricultural use for landfill. Waste biomass such as yard waste, forest trimmings, and timber operation wastes also accumulates in huge amounts and only a fraction of it is processed for value added products, with the major part being incinerated for energy or just discarded. But even left discarded, biomass, as a natural polymer decays quickly, but not so artificial, man-made polymers.
However, in the emerging new era of the natural resources shortage coupled with the vital necessity to reduce carbon footprint, solid polymeric wastes can be and should be considered a viable resource rather than a nuisance. Although recycling operations are implemented in many jurisdictions and municipalities, the recycled mix is limited, and certain types of potentially recyclable wastes are neglected. Even if recycling is in place, for example, as for scrap pneumatic tires, the amount of the waste recycled constitutes just a fraction of the total waste stream. It is estimated that about 300 million automotive tires are discarded every year in the United States, but only a fraction of that amount finds a second life. For example, in 2010, according to Institute of Scrap Recycling Industries, 50 million tires were processed to produce crumb rubber and an additional 27 million were used for road and environmental engineering. It is the urgent matter of sustainability to develop economically viable methods of recovering used polymers, to turn wastes into valuable feedstock.
The art needs new and improved methods for recycling waste polymeric matter. A desired method of processing waste cross-linked polymers should be energy and cost efficient. It is expected to produce ready to use products and generate no additional waste. These requirements have been implemented in the process described herein.